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Introduction to Hockey Analytics Part 4.2: The Various Forms of Corsi Available on Hockey Sites


Welcome back! In the last post a year ago, I discussed the value of possession metrics such as Corsi or Fenwick. As we noted back in that post, by keeping using shots - all shots, not just shots on goal, but missed and blocked shots as well - we can get an estimate of what percentage of time each team had possession of the puck (or well had it in the offensive zone as opposed to the defensive zone while a player was on the ice.

The most basic measure of possession in this manner is known as "Corsi." Corsi is - as detailed above - calculated by adding together shots on goal, missed shots and blocked shots by a player's own team and then subtracting the shots on goal, missed shots, and blocked shots of the opponent. In other words, just like +/- just using shots instead of goals and including both missed and blocked shots.

But as you might have noticed if you've gone to any hockey statistics site, you may have seen numerous statistics using the name "Corsi" and you may have become a little confused at what they all are. In addition, you may have seen statistics labeled as Corsi on different websites....but that appear to be different from each other.

So the idea of this post is to just provide a quick reference guide for all of you to ensure that you won't be confused as you try and use these possession statistics:

BehindTheNet.Ca

The most accessible website for hockey statistics these days is www.behindthenet.ca . BehindTheNet is terrific in that it provides you with access to most advanced hockey statistics for individual players and for teams for each of the last 5-6 years and even allows you to create a custom set of statistics so that you can have all of the statistics that you want on one single web page (for an example, here's an Islander custom report I use a lot:) . In fact you can even use this customization option to compare individual players over the last few seasons. But again, the site has a number of statistics with the name "Corsi" within them, so let's go over each to make sure that you're not confused.

Corsi On: The most basic measure - "Corsi On" simply measures the team's Corsi per 60 minutes while the player in question was on the ice (Hence: Corsi "on"). If a player's "Corsi On" is 5, it means that while that player was on the ice, his own team was outshooting (including missed and blocked shots remember) the opponent by 5 shots per 60 minutes.

Corsi Off: Corsi off is just what it sounds like: this is a measure of the team's corsi per 60 minutes while the player in question was OFF the ice. In other words, if we were talking about a certain player - let's say John Tavares - Tavares' Corsi ON would be the team's shot differential per 60 minutes while Tavares was on the ice and his Corsi OFF would be the team's shot differential when Tavares was sitting on the bench.

You won't need to look at Corsi Off much, but it's used to calculate the next statistic:

Corsi Relative: Corsi Relative is simply a measure of Corsi/60 minutes devised by subtracting Corsi Off from Corsi On. In other words, this statistic measures how much better a team's shot differential is WITH him on the ice than it is WITHOUT him. Thus, this is a measure that attempts to remove the influence of some teammates from a player's statistics. We've actually talked about this measure - more regularly called Relative Corsi - before back in Part 2.3 and covered it's strengths and weaknesses. In addition to Corsi on, Corsi Relative is a statistic you should be looking at to see how good a player is at driving possession.

Now, the Next Four Measures are not measures of a player's play, but are measures of Context:

Corsi QoC: "Corsi QoC" is short for Corsi Quality of Competition. We've talked about this before, and this is exactly as it sounds: this is a measure of the average Corsi On of opponents faced by the player in question.

Corsi Rel QoC: "Corsi Rel QoC" is short for Corsi Relative Quality of Competition. The same thing as Corsi QoC except using Corsi Relative rather than Corsi On. As I discussed in the previous post on Quality of Competition, I prefer this measure of QoC to Corsi QoC.

Corsi QoT: "Corsi QoT" is short for Corsi Quality of Teammates. We've talked about this before, and this is exactly as it sounds: this is a measure of the average Corsi On of teammates on the ice while the player in question is on the ice. As I noted in the previous post on QoT measures, I prefer this measure to the next measure:

Corsi Rel QoT: "Corsi Rel QoT" is short for Corsi Relative Quality of Teammates. The same thing as Corsi QoT except using Corsi Relative rather than Corsi On.

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TimeonIce.com

The next site you might be interested in getting possession statistics is TimeonIce.com. TimeonIce is nowhere near as easy to use as behindthenet, but it provides some really useful features that are well worth the time it takes to figure out the system.

On the most basic level, TOI provides possession statistics for each individual game, and the guys here at Lighthouse Hockey actually include a link to those numbers in every game's recap (They're just that cool). But really, individual game corsi #s aren't that useful - they're fun to gaze at (I'm very guilty of this myself, you might have noticed) but they're essentially useless for making any type of serious analysis - the sample size is just too small.

But through the use of TimeOnIce, you can find possession statistics for players and teams over any specific period of games. A very good guide as to this was created by RedArmyLine and can be found HERE. There is one specific thing regarding TOI.com that I'd like to note and that's how it presents it's possession statistics.

TimeOnIce presents it's possession statistics in percentage form. Take a look at the following TimeOnIce page, showing Islanders' stats from last season (note, it takes a while to load:):

John Tavares' Corsi % is listed as 0.506. What does this mean? Well it's a percentage - in other words, this number is telling you that 50.6% of shots (including missed and blocked shots) while John Tavares was on the ice were directed toward the opponent's net.

Why use percentages like this? Well remember, that Corsi essentially is a way of determining the amount of time that the team has control of the puck - or well has possession. So a Corsi % of .506 (or 50.6%) tells you that the Isles had possession of the puck 50.6% of the time when Tavares was on the ice. Again, higher numbers are better (obviously). Remember however, that TimeOnIce's numbers can't be viewed in a vacuum - you need to take context into account.

*TimeOnIce.com actually has one last feature which you can find in the RedArmyLine guide that I want to mention: It allows you to generate the possession #s for when the score is close (within 1 goal) or tied. This avoids score effects, which I've mentioned before in the last post.

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Conclusion:

There are other sites out there with this information - one other one is stats.hockeyanalysis.com, which has there own versions of the stats (to adjust by different levels of competition) as well as the ability to show the possession stats only in times where the score is tied or close. But behindthenet.ca and timeonice.com should be enough at the moment to service your corsi needs, and to allow you to figure out how well a player drives possession.

But there are two other stats in the vein of corsi that I need to discuss: Fenwick & Scoring Chances. That'll come in the next post.

The Intro to Hockey Analytics/Advanced-Hockey-Statistics Primer so far:

Part 1: - What is the field of Hockey Analytics and Why Might You be Interested?
Part 2.1: - The Importance of Context Part 1 - Time on Ice
Part 2.2: - The Importance of Context Part 2 - Evaluating the Difficulty of Certain TOI through QUALCOMP and Zone-Starts
Part 2.3: - The Importance of Context Part 3 - Evaluating (and Compensating for) the Effect of Teammates via QUALTEAM and Relative Measures
Part 2.4: - The Importance of Context Part 4: The Concept of the Replacement Level Player
Part 3 - The Perils of Sample Size
Part 4 - Introduction to Hockey Analytics Part 4.1: Possession Metrics (Corsi/Fenwick)

<em>Submitted FanPosts do not necessarily reflect the views of this blog or SB Nation. If you're reading this statement, you pass the fine print legalese test. Four stars for you.</em>

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